CS331 · Annotated SlidesChapter 2 · Part 2 — Physical Layer: Transmission Media
Chapter 2 · Part 2 · Annotated slides

Physical Layer: Transmission Media

Guided media (twisted pair, coaxial, fiber) and unguided media (radio, microwave, infrared): how each one is built, what it is good and bad at, UTP vs. STP and the cable categories, RJ-45 pinouts and when you need a crossover cable. Most questions here are comparisons, so the notes keep pointing at the differences.

Slide text Study notes Highlight: key words Tips, tricks & traps Seen again elsewhere How it can come up

Physical Layer

CS331: Computer Networks · Chapter 2, Part 2: Transmission Media
1
Exam weight Low
  1. Guided (wired): twisted pair, coax, fiber. Unguided (wireless): radio, microwave, infrared.
  2. Know one advantage and one disadvantage of each, plus the frequency ranges of the three wireless types.

Transmission medium and the physical layer

  • Transmission medium(channel): the physical path between the transmitter and the receiver.①
  • Located below the physical layer
  • Controlled by the physical layer②
Sender’s physical layer → transmission medium (cable or air) → receiver’s physical layer
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Exam weight Medium
  1. The medium is BELOW the physical layer (sometimes called layer 0) and is controlled by it.

Seen again

↻ Ch 1 Part 1, slide 3 · The medium as a component of data communication
🎯 How this can come up · 1 question
True / False

The transmission medium is located above the physical layer.

Classes of transmission media

Transmission media: guided (wired) twisted pair, coaxial, fiber-optic; unguided (wireless) radio wave, microwave, infrared
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Exam weight High
  1. Guided = wired: twisted pair, coaxial, fiber-optic.
  2. Unguided = wireless: radio wave, microwave, infrared.
Memory trickGuided = the signal is GUIDED along a physical path. Unguided = it just spreads through the air.
🎯 How this can come up · 2 questions
MCQ

Which is an unguided medium?

MCQ

Fiber-optic cable is classified as:

Guided media

  • Twisted-pair cable
  • Coaxial cable
  • Fiber-optic cable
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Exam weight Low
  1. Three guided media, from cheapest and slowest to most expensive and fastest.

Twisted pairs

Colour-coded twisted wire pairs in a cable
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Exam weight Low
  1. Four pairs, each a different colour, inside one jacket.

Twisted-pair cable

  • A twisted pair consists of: two insulated copper wires in a regular

spiral pattern①

  • A wire pair acts as a single

communication link

  • Twisted to reduce electrical

interference from similar pairs close by (more twists means better quality)②

  • Used in:
    • Telephone network Between house and local exchange (subscriber loop)
    • LAN③
Without twisting, interference reaches the pair; with twisting the interference cancels (less crosstalk)
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Exam weight High
  1. Two insulated copper wires twisted together; one pair = one link.
  2. Twisting reduces interference (crosstalk) from neighbouring pairs. More twists = better quality.
  3. Used in the telephone subscriber loop and in LANs.
🎯 How this can come up · 2 questions
MCQ

Why are the wires in a twisted pair twisted?

True / False

More twists per length gives better quality twisted-pair cable.

Twist length: Category 3 vs. Category 5 UTP

(a) Category 3 UTP: twist length of 7.5 cm to 10 cm

(b) Category 5 UTP: twist length 0.6 cm to 0.85 cm

Cat 3 has long twists, Cat 5 has short, tight twists
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Exam weight Low
  1. Cat 5 is twisted far more tightly than Cat 3, so it carries higher frequencies with less crosstalk.

Why twisting cancels interference

a) Two parallel wires: the wire nearer the noise source picks up more, so a difference (noise) remains.

b) Twisted pair: the interference affects both wires equally overall, so the unwanted radiation is largely canceled.①

Parallel wires get a difference of +6; twisted wires get a difference of 0
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Exam weight Medium
  1. The receiver looks at the DIFFERENCE between the two wires. Twisting makes the noise equal on both, so it subtracts out.
🎯 How this can come up · 1 question
Written

Explain how twisting a pair of wires reduces noise.

UTP and STP cables

UTP: Unshielded Twisted Pair

STP: Shielded Twisted Pair

a. UTP: pair inside a plastic cover. b. STP: pair wrapped in a metal shield inside a plastic cover
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Exam weight Low
  1. STP adds a metal shield (foil or braid) around the pairs.

STP vs. UTP

Shielded Twisted Pair (STP)

  • Metal braid or covering that reduces interference
  • More expensive
  • Harder to handle (thick, heavy)①

Unshielded Twisted Pair (UTP)

  • Cheapest type of cable
  • Easiest type to install
  • Suffers from external electromagnetic (EMI) interference②
Photos of shielded and unshielded twisted pair
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Exam weight High
  1. STP: shield → less EMI, but more expensive and harder to handle.
  2. UTP: cheap and easy, but suffers from EMI. UTP is what most LANs use.
Exam trapUTP is not “worse” for every case: it is the default in LANs because it is cheap and easy.
🎯 How this can come up · 2 questions
MCQ

Compared with UTP, STP:

Scenario

A factory floor has heavy motors next to where the network cables will run. UTP or STP, and why?

Categories of unshielded twisted-pair cables

  • CAT5e: 100 MHz · 1000 Mbps · UTP or STP①
  • CAT6: 250 MHz · 1000 Mbps · UTP or STP
  • CAT6a: 500 MHz · 10 Gbps · UTP or STP
  • CAT7: 600 MHz · 10 Gbps · shielded only
  • CAT8: 2000 MHz · 25 Gbps or 40 Gbps · shielded only

Shielded CAT8: up to 40 Gbps over 24 m②

A shielded CAT8 cable
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Exam weight Medium
  1. Higher category = higher bandwidth (MHz) and data rate.
  2. From CAT7 up the cable must be shielded.
🎯 How this can come up · 2 questions
MCQ

Which is the first category in the table that is shielded only?

MCQ

CAT6a supports a maximum data rate of:

UTP connectors

RJ = Registered Jack

RJ-45 female socket, RJ-45 male plug, and photos of cables
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Exam weight Low
  1. RJ-45 (Registered Jack) is the 8-pin connector of twisted-pair Ethernet.

The RJ-45 connector pinout

  • Pin 1 TD+: Transmit Data positive · Pin 2 TD-: Transmit Data negative
  • Pin 3 RD+: Receive Data positive · Pin 6 RD-: Receive Data negative①
  • Pins 4, 5, 7, 8 N/C: No Connection / Not Connected → unused pins②
RJ-45 plug with pins 1–8: 1 TD+, 2 TD-, 3 RD+, 6 RD-, 4/5/7/8 not connected
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Exam weight Medium
  1. 10/100 Ethernet uses only 4 pins: 1 & 2 transmit, 3 & 6 receive.
  2. 4, 5, 7, 8 are unused (Gigabit Ethernet uses all 8).
🎯 How this can come up · 2 questions
MCQ

In 10/100 Ethernet, which RJ-45 pins are used to transmit?

MCQ

Which pins receive data?

Crossover cable

In a crossover cable, one end’s transmit pins (1, 2) are wired to the other end’s receive pins (3, 6).

Two RJ-45 ends with pairs 1/2 and 3/6 swapped
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Exam weight Low
  1. Crossover swaps 1↔3 and 2↔6, so TX on one side meets RX on the other.

Ethernet cabling: straight-through vs. crossover

Straight-through cabling

  • PC to switch/hub
  • Router to switch/hub

Pins 1→1, 2→2, 3→3, 6→6①

Crossover cabling

  • PC to PC
  • Hub to hub
  • Switch to switch
  • Hub to switch

Pins 1→3, 2→6, 3→1, 6→2②

Straight-through: pins 1–1 to 6–6. Crossover: pins crossed; PC to PC needs a crossover or TX meets TX
15
Exam weight High
  1. Different kinds of device (PC ↔ switch, router ↔ switch) → straight-through.
  2. Same kind of device (PC ↔ PC, switch ↔ switch, hub ↔ hub, hub ↔ switch) → crossover, or TX meets TX.
Memory trick“Unlike → straight, like → cross.” (Router ↔ PC counts as “like”: both are end devices.)
🎯 How this can come up · 3 questions
MCQ

Which cable connects two PCs directly?

MCQ

Which cable connects a PC to a switch?

True / False

Two switches are connected with a straight-through cable.

Coaxial cable

  • Inner copper wire (the conductor) inside insulation
  • Outer conductor of copper mesh (braid or foil) that shields it
  • Outer jacket
Coaxial cable: copper wire, insulation, copper mesh, outer jacket
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Exam weight Low
  1. Coax = a central copper wire + an outer copper mesh as a shield. Used in cable TV and old bus Ethernet (10Base5, 10Base2).

Seen again

↻ Ch 4, slide 18 · Which media each Ethernet generation uses

Optical Fiber

  • Consists of three concentric sections: fiber core and cladding, a glass or plastic cover, and a plastic jacket
    • Core: one or more very thin strands or fibers made of glass or plastic①
    • Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from the core
    • Jacket: a plastic or other material acts as a layer to protect against moisture, crushing, and other environmental dangers.②
Fiber layers: core, cladding, buffer, outer jacket
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Exam weight Medium
  1. Core (carries the light) → cladding (different optical property, reflects the light back in) → jacket (protection).
🎯 How this can come up · 1 question
MCQ

The layer of a fiber that keeps the light inside the core is the:

Optical Fiber

Light bounces between the cladding walls as it travels down the core from sender to receiver
18
Exam weight Low
  1. Light travels by reflecting off the core/cladding boundary.

Fiber Optics Properties

Advantages

  • Higher bandwidth: data rates of hundreds of Gbps
  • Smaller size & weight
  • Lower attenuation (signal loss)
    • Greater repeater spacing: can run 50 km at least without repeaters①
  • No crosstalk (no light leaking)
  • Not affected by noise
  • Highly secure (no light leaking)

Disadvantages

  • Needs special skills to install
  • Cost: more expensive②
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Exam weight High
  1. Fiber: more bandwidth, less attenuation (50+ km without repeaters), immune to noise and crosstalk, very secure.
  2. Downsides: expensive, needs skilled installers.
Exam trapFiber is NOT cheaper or easier to install. Those are its two disadvantages.
🎯 How this can come up · 3 questions
MCQ

Which is a disadvantage of fiber-optic cable?

True / False

Fiber-optic cable is not affected by electromagnetic noise.

Written

Give three advantages of fiber over copper.

UTP cabling vs. fiber-optic cabling

UTP cabling

  • Bandwidth: 10 Mb/s – 10 Gb/s
  • Distance: relatively short (1 – 100 m)
  • Immunity to EMI and RFI: low
  • Immunity to electrical hazards: low
  • Media and connector cost: lowest
  • Installation skills: lowest①

Fiber-optic cabling

  • Bandwidth: 10 Mb/s – 100 Gb/s
  • Distance: relatively long (1 – 100,000 m)
  • Immunity to EMI and RFI: high (completely immune)
  • Immunity to electrical hazards: high (completely immune)
  • Media and connector cost: highest
  • Installation skills: highest②
20
Exam weight High
  1. UTP: up to 100 m, cheap, easy, but not immune to EMI.
  2. Fiber: up to 100 km, immune to EMI and electrical hazards, but the most expensive.
🎯 How this can come up · 2 questions
MCQ

The maximum distance of a UTP segment in the table is about:

Scenario

Two buildings 2 km apart must be linked, and a high-voltage line runs between them. Which cable?

Unguided media: wireless

  • Radio waves
  • Microwaves
  • Infrared
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Exam weight Low
  1. Three unguided media, from lowest to highest frequency.

Wireless transmission waves (Figure 7.19)

Unguided media: signals are transmitted through air and are available to everyone who has a device that can receive them①

Wireless transmission splits into radio wave, microwave and infrared
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Exam weight Low
  1. Unguided: the signal goes through the air and anyone with a receiver can pick it up (security matters).

Electromagnetic spectrum for wireless communication

As we move from left to right, the frequency increases.

Radio waves and microwaves: 3 kHz – 300 GHz · Infrared: 300 GHz – 400 THz · then visible light (400 – 900 THz)①

Spectrum from radio and microwave through infrared to visible light
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Exam weight Medium
  1. Radio < 300 MHz < microwave < 300 GHz < infrared < 400 THz < visible light.
Memory trickOrder by frequency: Radio → Microwave → Infrared → Light (“Really Many Infrared Lights”).
🎯 How this can come up · 1 question
MCQ

Which has the highest frequency?

Radio waves

  • Less than 300 MHz①
    • Omnidirectional (signal propagates in all directions)
    • Can Travel long distances
    • Broadcast radio (AM, FM), TV②
An omnidirectional antenna radiating in all directions
24
Exam weight Medium
  1. Radio: below 300 MHz, omnidirectional, long distances, passes through walls. Used for AM/FM and TV.

Note: radio waves

Radio waves are used for multicast communications, such as radio and

television

25
Exam weight Medium
  1. Radio = MULTICAST / broadcast (one sender, many receivers: radio and TV).
🎯 How this can come up · 1 question
MCQ

Radio waves are used for which type of communication?

Microwaves

  • 300MHz to 300GHz (sub range from the Radio waves)①
    • Travel in one direction (Unidirectional = line-of-sight propagation = straight lines)
    • Can be used for point to point for example between buildings to connect their LANs
    • high frequency microwaves, difficult to pass walls (if receivers inside buildings)
      • Higher frequency → shorter wavelength → more difficulty passing through obstacles such as walls.②
    • Used in Wireless networks, satellite, cellular phones.
      • microwaves carry information through the air.③
26
Exam weight High
  1. Microwave: 300 MHz – 300 GHz, unidirectional (line of sight), can link two buildings point to point.
  2. Higher frequency → shorter wavelength → harder to pass through walls.
  3. Used in wireless networks, satellite and cellular phones.
Exam trapRadio is OMNIdirectional; microwave is UNIdirectional (line of sight).
🎯 How this can come up · 2 questions
MCQ

Microwaves propagate:

True / False

Higher frequencies pass through walls more easily.

Note: microwaves

Microwaves are used for unicast communication such as cellular

telephones, satellite networks,

and wireless LANs.

27
Exam weight Medium
  1. Microwave = UNICAST: cellular phones, satellite networks, wireless LANs.
🎯 How this can come up · 1 question
MCQ

Cellular telephones and satellite networks mainly use:

Infrared (IR Wireless)

  • 300GHz to 400THz①
    • Have a very large bandwidth
    • Local- short distance communication because they cannot penetrate walls
    • Line-of-sight propagation (directional)
    • Used in local point-to-point Transmission
    • Applications: remote controls, some wireless headphones, and some wireless computer peripherals.②
    • Use Infrared Data Association (IrDA) protocol
    • Cannot be used under the sun because of the interference with the sun infrared rays
    • Replaced by Bluetooth Technology③
28
Exam weight High
  1. Infrared: 300 GHz – 400 THz, very large bandwidth, line of sight, short range (can’t pass walls).
  2. Remote controls and some peripherals (IrDA protocol). Doesn’t work in sunlight. Mostly replaced by Bluetooth.
🎯 How this can come up · 2 questions
MCQ

Why can’t infrared be used outdoors in the sun?

MCQ

Which wireless medium cannot penetrate walls and is used in TV remote controls?

Note: infrared

Infrared signals can be used for short-range communication in a closed area

using line-of-sight propagation.

29
Exam weight Medium
  1. Infrared = short range, closed area, line of sight.
🎯 How this can come up · 1 question
Written

Compare radio waves, microwaves and infrared by direction and typical use.